The cost of corrosion to the United States is conservatively estimated at $300 billion each year. In addition to the tremendous cost in dollars, corrosion causes catastrophic events that have led to loss of human life through collapse of structures such as bridges. Tens of millions of dollars are invested each year into research for solutions to this problem. This research has only produced technologies that delay the onset of corrosion. The problem of preventing corrosion remains largely unsolved.
Current anti-corrosion products are predominantly various types of polymeric coatings, rust inhibitory chemicals embedded in an overcoat, and recently various alloys that slow the onset of corrosion. Current coatings and overcoats suffer from many problems including imperfect application, non-uniform material, insufficient adhesion, and environmental deterioration. Alloys are expensive and environmentally-sensitive, which affects the choice of for a particular application.
The automobile industry has attempted to find a successful anti-corrosion application, as cost to this industry accounts for one-third of total corrosion costs in the United States. Repairs for corrosion damage to federal bridges in the United States alone cost $50 billion annually and more than half of these bridges require major work due to corrosion. Estimates place the overall worldwide cost to repair reinforced concrete structures at $200 per square meter of exposed surface area. The aircraft and shipbuilding industries are also significantly affected by corrosion. Billions of dollars could be saved with an effective anti-corrosion technology.
Corrosion is an electrochemical process defined as the destructive attack of a metal by the environment. Factors causing corrosion include oxygen concentrations in the water and the atmosphere, hydrogen sulfide, acid rain, temperature, surface texture of metals, concentrations of various salts in solution or in the air, pH of the environment, microorganisms, and biological and industrial contaminants. These factors affect corrosion in the following ways.
The presence of water mixed with contaminants is the basis of galvanic corrosion. Pure rain water is slightly acidic (pH 5.5 to 6.0). It mixes with carbon dioxide and creates carbonic acid. Carbonic acid can attack some metals, including copper. The ions etched from the copper go into solution in the rain water. This leaves the bare steel to oxidize. Metals are subject to solution pressure when immersed in solutions of their metallic salts. Metals may be arranged in order of their solution pressure. When two metals are exposed to water, one may become anodic, suffer corrosion and affect the other metal by rendering it cathodic.
Oxygen is the main accelerator of corrosion. Rainwater picks up oxygen as it falls through the atmosphere. Fresh and salt water are excellent carriers of oxygen. Both bacteria and fungi can deteriorate metal. They produce acids that etch metals, and also produce acids from their decomposition.
There are different types of corrosion: galvanic cells in which corrosion operates between two dissimilar metals; salt concentration cells which are common where metal is in contact with two concentrations of the same salt; and oxygen concentration cells which are differential aeration cells in which corrosion is accelerated at the low oxygen concentration site by an electrochemical process because the high concentration site is cathodic. This is primarily due to the fact that negative free energy of formation of a metal oxide indicates a tendency for the metal to react, therefore the oxide is unstable, but positive free energy indicates that the metal is stable.
Corrosion may be divided into the following categories: 1) Uniform Etch--A direct chemical attack from salts and acids. If this is allowed to continue, a polished surface will dull and then present a rough or frosted appearance; 2) Pitting--Tiny pin holes from localized chemical or galvanic attack; 3) Intergranular--Usually galvanic, this is a selective attack along the grain boundaries of an alloy metal. We have referred to this as "de-alloying". Intergranular corrosion is a form of stress-corrosion cracking which is the spontaneous failure of metal or alloy as a result of the combined action of stress and corrosion. Stress-corrosion cracking produces two types of failure which can be classified as intergranular and transgranular cracking. Intergranular failure occurs when the cohesion between the crystallographic grains of the metal is reduced appreciably by chemical attack and cracking follows predominantly the grain boundaries. Transgranular failure results when cleavage of the grains occurs. Typical corrosion-resistant alloys can break down when corrosion works on the individual components of the alloy; 4) Exfoliation is localized subsurface corrosion in zones parallel to the surface which result in thin layers of uncorroded metal. Found on extruded metals, this corrosion occurs just below the metal surface and causes a blister to form. This blister appears where the extruding dies have forced the crystal structure of the metal to change direction; 5) Galvanic--The classic two dissimilar metal connection with a water electrolyte bridge is the most basic of corrosion problems; 6) Concentration Cell--As the amount of oxygen reaching the electrolyte varies, the rate of corrosion will vary accordingly. Highly concentrated areas of oxygen will display high levels of corrosion; 7) Stress--More corrosion will occur where high tensile stress is applied. This stress occurs where metal is bent or where rivets have been driven. Metals, such as copper, that have been cold worked (bent back and forth several times) should be annealed (stress relieved by heating). Stress corrosion appears as a cracks running parallel to the metal grain boundaries; 8) Fatigue--Fatigue is another form of stress corrosion where pits are defined along the grain. Additional stress begins to concentrate around them and cracking occurs at the bottom of the pits; and 9) Filiform--Thread-like filament corrosion occurrs under painted surfaces where water and oxygen have penetrated and formed a corrosion concentration cell.
Current polymeric coatings have not adequately addressed the problem of corrosion. Conditions at cathodic areas become alkaline, necessitating that protective paints have good alkali resistance. The resistance of a paint film is affected by the presence of electrolytes in the film, and hence water-soluble impurities in pigments must be kept to a minimum. The presence of electrolytes beneath the film mandates surface cleanliness prior to paint application. Penetration of the film by water and electrolytes from the outside requires highly cross-linked films to become more resistant in general to such penetrations. On the basis of these considerations, it is clear that the essential requirements for protective paint coatings are that they should be applied to a clean metal surface, they should provide a high electrical resistance between the metal and its environment, and they should withstand alkaline conditions.
Recently, conductive plastics have been examined by various industries. Plastics do not generally corrode in the same manner as metals. This is primarily based on the fact that the coatings act as oxidation-reduction (redox) chemicals that approach those of noble metals, therefore inhibiting corrosion.
Inhibitors are chemical substances that, when added in small amounts to the environment in which the metal would corrode, will retard or entirely prevent this corrosion. However, an inhibitor must be absorbed or bonded to the metal surface for it to be effective. The type of bond absorption varies with the chemical configuration of the inhibitor molecule. Other problems faced when using inhibitors include their instability in air and the rapid decomposition of the polymer. Some active species from the environment can be absorbed into the plastic to swell or react internally with the polymer chains. Softening and distortion normally develop, although actual loss of weight from the plastic can occur. In addition, the high cost of production of inhibitors also creates the need to investigate alternative options.
Alloys have been used extensively but are expensive and hazardous to the environment. Current chromium electroplating methods will become illegal and virtually eliminated by the year 2000. Numerous federal, state, and local regulations have been established because of the large quantity of potentially toxic chemicals used in this process. These regulations will force over 11,000 metal finishing companies to find alternative methods that provide similar properties as offered by chromium.
Oxidation of the molecules in plastic can occur in the atmosphere or other oxidizing conditions. This often results in hardening and cracking of the plastic. In addition, continued polymerization of the resin can occur with certain resinous components resulting in hardening, shrinkage, and cracking of the material.
The important aspect of the corrosion mechanisms in plastics lies in the fact that degradation of plastic is not a surface effect like metallic corrosion, but occurs internally. A plastic material may absorb parts per million (ppm) of an aggressive agent from an otherwise innocuous source and potentially result in a total loss of mechanical properties.
Accordingly, what is needed are compositions and methods for preventing corrosion.
What is also needed are compositions and methods for preventing corrosion that are inexpensive.
What is also needed are compositions and methods for preventing corrosion that minimize the use of volatile organic compounds (VOCs).